The invention relates to an exhaust gas recirculation controller that recirculates a part of exhaust gas in a combustion engine into an intake pipe of the combustion engine. More particularly, it is directed to a troubleshooting system for the exhaust gas recirculation controller.
An exhaust gas recirculation (EGR) controller (hereinafter referred to as "EGR controller") has heretofore been in wide use as a means for reducing NOx (nitrogen oxide) contained in exhaust gas within a combustion engine. The EGR controller controls EGR by a back pressure control system employing a back pressure transducer (BPT) valve. That is, the passage area of an EGR control valve (hereinafter referred to as "EGR valve") is controlled by the BPT valve so that the EGR flow amount can be maintained at a predetermined value. Even in a system using a venturi vacuum transducer (VVT) valve as well as in a system for controlling the control pressure of the EGR valve using a solenoid valve, the area of passage of the EGR valve is similarly controlled as in the system using the BPT valve.
By the way, a system for troubleshooting such an EGR controller has been proposed in Japanese Patent Unexamined Publication No. 51746/1987. The system is designed to detect the pressure of air sucked into an intake manifold when there exists an EGR flow amount (when EGR is turned on), i.e., an intake manifold pressure value P.sub.ON (see FIG. 4), and an intake manifold pressure value P.sub.OFF when there exists no EGR flow amount (when the EGR is turned off) (see FIG. 4). And the system issues an alarm when the difference between these pressures is out of a predetermined range.
However, thus constructed conventional EGR controller that controls the EGR flow amount by varying the passage area of the EGR valve has addressed the following problems.
(1) Since a stable intake manifold pressure after the EGR has been turned on or off must be measured (by "stable," it is intended to mean that a change has been complete with all transients and fluctuations settled down), a relatively long time T.sub.B (see FIG. 4) during which the operation is stable must be provided. This has been responsible for relatively small troubleshooting chance. PA0 (2) Since a stable intake manifold pressure after the EGR has been turned off must be measured, a relatively long period T.sub.2 (see FIG. 4) during which the EGR is turned off must be provided. This has imposed a driveability problem that an operator is subjected to mechanical shock. Further, the long EGR-off time deteriorates exhaust gas conditions. PA0 (3) In a region where the amount of air sucked is small, the EGR flow amount is naturally small. Thus, the difference in intake manifold pressure between the turning on and off of the EGR becomes small, thereby making the troubleshooting difficult. Further, when the amount of air sucked is large (or the intake manifold pressure (absolute pressure) is high, or the back pressure (absolute pressure) is high), the difference in EGR flow amount between the turning on and off of the EGR becomes large, but the difference in intake manifold pressure becomes small while affected by the pressure loss in the recirculation pipe due to the large EGR flow amount. Thus, the troubleshooting is also difficult. PA0 (4) Since the EGR flow amount changes abruptly as the EGR turns on or off (see parts a, b in FIG. 25 (c)), the fixing of the amount of air to be supplied to the internal combustion engine (ISC air amount) (see FIG. 25 (d)) at a constant value causes firing conditions to be changed between the turning on and off of the EGR, which then causes variations in torque (see parts c, d in FIG. 25 (e)). Particularly, since the variations in torque at the time the EGR turns on and off are larger in a region with a large EGR flow amount, there is the driveability problem that the operator is subjected to mechanical shock.